1. Technical Field
The invention relates to an image processing apparatus and an image processing method for generating an image signal representing gradation using an area coverage modulation method.
2. Related Art
An image formation apparatus such as a copier or a printer using electrophotography, an ink jet system, etc., performs for input image information a screen process of artificially reproducing a halftone with binary values representing presence or absence of a drawing dot, and outputs the result by means of an output device.
A document generated by such an image formation apparatus includes various objects from the viewpoint of the image quality, such as an image of a photo, etc., graphics of gradation, patch, etc., and text, a line, etc.,
The number of lines per inch (how many identical patterns are contained per inch) is a representative parameter of the screen process. The optimum number of lines for the image object varies from one image object to another.
For example, preferably an image is generated with 200 lines per inch from the viewpoint of the human's eye characteristic. However, 170 to 230 lines per inch are often used because the gradation property and graininess are degraded due to the responsiveness of a marking engine if the number of lines per inch is high and because a screen structure is seen or texture is lost if the number of lines per inch is low.
For graphics, importance is further placed on the gradation property and graininess. Thus, 150 to 200 lines per inch are often used, and 300 lines or more per inch are often used for text and a line, which are desired to have no jaggy and no break.
That is, preferably the number of lines per inch is high as much as possible to smoothly represent text, a line, etc., but the gradation property and graininess are degraded if the number of lines per inch is too high.
Thus, the screen process is performed with a different number of lines per inch in response to each object. That is, plural threshold matrices different in the number of lines per line are stored and the screen process is performed with a different number of lines per inch in response to an object using the threshold matrices.